1. Field of the Invention
The present invention relates to mirrors, and more particularly, to a method of adhering silver to glass.
2. Description of the Prior Art
Mirrors, which are light-reflective surfaces, have been most commonly made over the past century by providing a metallic or amalgam backing on a glass surface. A silver backing is preferred and is capable of providing reflection of the highest quality.
The conventional state-of-the-art methods of coating silver layers on glass surfaces prior to this invention generally fall into three categories, including wet chemical electroless processes, organometallic processes, and vacuum deposition. There are other well-known methods of plating glass surfaces with silver, such as electrolysis; however, the costs of such other processes are prohibitive for commercial use.
Each of the conventional prior art methods of coating a glass surface has specific advantages and disadvantages. However, they have been used successfully and to great advantage for many years in producing high quality mirrors, most of which in the past have been used indoors. With the advent of increased interest in the use of mirrors outdoors for solar collectors, such as heliostats and other solar collector applications over the past decade, the disadvantages and deficiencies of mirrors produced by these conventional methods have become more significant and more acute.
For example, many conventional silvered-glass mirrors are in the form of composite structures. These structures usually consist of a substrate glass layer or pane, a thin reflector layer of silver on the glass surface, a layer of copper over the silver layer, and a protective layer of a paint substance comprised primarily of polymers, carbon, lead, barium, titanium, and other heavy metals. While this kind of mirror structure is satisfactory for many years of service indoors, the reflectance and other desirably optical properties of these mirror structures degrade when placed in a terrestrial environment outdoors. In particular, silver is susceptible to agglomeration, the formation of oxides, sulfides, and chlorides, diffusion between component layers, and de-adhesion or separation of the silver layer from the glass substrate. One example of such conditions involves environmental gases or pollutants, such as combinations of moisture and oxygen, hydrogen sulfide (H.sub.2 S), sulfur dioxide (SO.sub.2), and/or hydrogen chloride (HCl), which diffuse through the layers of paint to react with the copper and the silver reflector layer. Electrochemical reactions also occur at the interface region of the silver reflector layer and the glass substrate, as well as at the side edges of the composite mirror structure. The degradation typically occurs within a few months to a few years, which is an unacceptably short time when a desired lifetime for such equipment is 20 to 30 years.
Although the silvered mirrors produced by the wet chemical electroless processes and by the organometallic methods are susceptible to chemical degradation resulting in loss of reflectivity, the mirrors produced by vacuum deposition processes, such as sputtering and vacuum evaporation, which generally have superior optical qualities and fewer impurities, are more susceptible to de-adhesion of the silver layers from the glass so that the silver layers flake or peel off the glass substrate surfaces. Because of the very large surface areas of mirrors required for commercial scale heliostats and other solar collectors, it is necessary to increase the durability of mirrors manyfold before such installations become economically feasible.